Vercel externals

[Rich-Harris]

fixes #3588, by adding an external option that gets passed to esbuild

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Latest commit: 86a7ea2

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@sveltejs/adapter-vercel	Patch

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[bluwy]

I wonder if it's worth having the param as plugin(options?: Options) instead to simplify the type signature for intellisense, and conforms with the docs wording of "You can pass an options argument". But this PR is fine with the current type too.

[Rich-Harris]

make sense, i made that change